Method and apparatus to control handoff between different wireless systems

ABSTRACT

A wireless communications network includes a first base station system that performs wireless communications according to a first protocol (e.g., 1xEV-DO protocol) and a second base station system that performs wireless communications according to a second, different protocol (e.g., IS-2000). A link is provided between the first and second base station systems to enable a network-initiated handoff procedure. If a source base station system detects that a handoff of a mobile station to a target base station system is required, the source base station system exchanges messaging over the link with the target base station system to perform the handoff. In one example, the handoff is a hard handoff.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Ser. No. 09/960,008, entitled“Method and Apparatus to Control Handoff Between Different WirelessSystems,” filed Sep. 21, 2001 which claims the benefit under 35 U.S.C.§119(e) of U.S. Provisional Application Ser. No. 60/251,492, filed Dec.4, 2000, both of which are hereby incorporated by reference.

TECHNICAL FIELD

The invention relates generally to methods and apparatus for controllinghandoffs between different wireless systems.

BACKGROUND

Mobile communications systems, such as cellular or personalcommunications services (PCS) systems, are made up of a plurality ofcells. Each cell provides a radio communication center in which a mobilestation establishes a call with another mobile station or a wirelineunit connected to a public switched telephone network (PSTN) or a packetswitched data network (PSDN). Each cell includes a radio base station,with each base station connected to a mobile switching center thatcontrols processing of calls between or among mobile stations or mobilestations and wireline terminals.

Various wireless protocols exist for defining communications in awireless network. One such protocol is the time-division multiple access(TDMA) standard, such as the TIA/EIA-136 standard provided by theTelecommunications Industry Association (TIA). Another TDMA-basedwireless communications technology is the Global System for Mobile (GSM)technology. According to TDMA, each radio frequency (RF) carrier carriesa frame that is divided into plural (e.g., six or eight) times slots toincrease the number of mobile stations that can be supported per RFcarrier.

Another standard for wireless communications is the code-divisionmultiple access (CDMA) standard, with versions implemented as the IS-95Aand IS-95B standards. CDMA is a spread spectrum wireless communicationsprotocol in which transmission is based on the spread spectrummodulation technique to allow many users to have access to the same bandof carriers.

Traditionally, wireless networks have been designed for carryingcircuit-switched voice traffic. However, with the explosion of theInternet and intranets, packet-switched communications (e.g., webbrowsing, electronic mail, and so forth) have become common. As aresult, third generation (3G) wireless technologies are being developedto transition to higher bandwidth and more efficient packet-switchedcommunications (of data as well as voice and other forms of real-timedata) over wireless networks.

To achieve 3G requirements, the CDMA 2000 family of standards has beendeveloped, also referred to as the IS-2000 standards. A CDMA 2000wireless communications system is capable of supporting both traditionalvoice traffic as well as packet-switched traffic, such as web browsing,electronic mail, voice-over-IP (Internet Protocol), and so forth. On theTDMA side, packet-switched wireless communications protocols have alsobeen developed, including the Enhanced Data Rate for Global Evolution(EDGE) technology, sometimes referred to as Enhanced GPRS (GeneralPacket Radio Service).

The first phase of CDMA 2000 is referred to as 1xRTT (also referred toas 3G1X or 1×), which is designed to increase voice capacity as well asto support data transmission speeds that are faster than typicallyavailable. In addition, for even higher data rates, a High Data Rate(HDR) wireless technology has been developed. HDR is defined asTIA/EIA/IS-856, “CDMA 2000, High Rate Packet Data Air InterfaceSpecification,” which is adopted by the TIA. The HDR technology is alsoreferred to as the 1xEV-DO or 1xEV technology. 1xEV-DO providesrelatively high data transfer rates over the air interface betweenmobile stations and base stations (referred to as access networks or ANsby 1xEV-DO).

For mobile stations that are capable of working in both 1xRTT and1xEV-DO systems, a graceful way has not been developed for transitioninga mobile station between a 1xEV-DO system and a 1xRTT system. Onecurrent scheme that exists for transitioning a mobile station between1xEV-DO and 1xRTT systems is that when a mobile station transitions fromone system to another, the call is actually dropped (or terminated) inthe system that the mobile station is leaving and re-originated in thesystem that the mobile station is entering.

A need exists for an improved method and apparatus of transitioningmobile stations between different wireless systems, such as 1xRTT and1xEV-DO systems.

SUMMARY

In general, according to one embodiment, a method of performing wirelesscommunications comprises communicating bearer traffic for apacket-switched communications session between a mobile station and afirst base station associated with a first type of wireless system, anddetermining if handoff is required from the first base station to asecond base station associated with a second, different type of wirelesssystem. Messages are exchanged between the first and second basestations to perform the handoff in response to determining that thehandoff is required.

Other or alternative features will become apparent from the followingdescription, from the drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example communications network thatincludes a wireless network.

FIG. 2 is a message flow diagram for handing off a call between a1xEV-DO system and a 1xRTT system.

FIG. 3 is a block diagram of a platform or platforms including a basestation or access network and a packet control function (PCF) module.

DETAILED DESCRIPTION

In the following description, numerous details are set forth to providean understanding of the present invention. However, it will beunderstood by those skilled in the art that the present invention may bepracticed without these details and that numerous variations ormodifications from the described embodiments may be possible.

Referring to FIG. 1, a wireless communications network 10 has a coveragearea designated generally as 12. In one embodiment, the wirelesscommunications network 10 includes components that operate according toCDMA (code-divisional multiple access) 2000. CDMA 2000 is defined by theCDMA 2000 family of standards (collectively referred to as the IS-2000Standard), which is developed by the Third Generation PartnershipProject 2 (3GPP2). However, in other embodiments, other types ofwireless protocols can be used for communications in the wirelesscommunications network 10.

The wireless communications network 10 includes a base station (BS) 14,which is an entity used for radio telecommunications with mobilestations (e.g., mobile station 16) within a cell 18 covered by the basestation 14. The radio equipment of the base station 14 is implemented ina base transceiver subsystem (BTS) 20, and the control portion of thebase station 14 is the base station controller (BSC) 22.

For communicating circuit-switched voice traffic, the base station 14 iscoupled to a mobile switching center (MSC) 24, which is responsible forswitching mobile station-originated or mobile station-terminatedtraffic. Effectively, the MSC 24 is the interface for signaling and usertraffic between the wireless network 10 and other public switchednetworks (such as a public switched telephone network (PSTN) 26) orother MSCs. The PSTN 26 is connected to landline terminals, such astelephones 28.

In a voice call session between a mobile station (such as mobile station16) and a landline terminal (such as telephone 28), voice traffic isrouted through the air interface between the mobile station 16 and abase station 14, and through the base station 14, MSC 24, and PSTN 26.

The wireless communications network 10 also supports packet dataservices, in which packet data is communicated between a mobile stationand another endpoint, which can be a terminal coupled to a data network34 or another mobile station that is capable of communicating packetdata. Examples of the data network 34 include private networks (such aslocal area networks or wide area networks) and public networks (such asthe Internet). Packet data is communicated in a packet-switchedcommunications session established between the mobile station and theother endpoint.

To communicate packet data, the base station 14 is coupled to a packetcontrol function (PCF) module 32, which manages the relay of packetsbetween the base station 14 and a packet data serving node (PDSN) 30.The base station 14 and PCF module 32 can be implemented on one platformor on multiple platforms. An interface 15 between the base station 14and the PCF module 32 may include an A8 interface to carry user trafficand an A9 interface to carry signaling. An interface 31 between the PCFmodule 32 and PDSN 30 may include an A10 interface to carry user trafficand an A11 interface to carry signaling.

The PDSN 30 establishes, maintains, and terminates link layer sessionsto mobile stations, and routes mobile station-originated or mobilestation-terminated packet data traffic. The PDSN 30 is coupled to thepublic data network 34, which is connected to various endpoints, such asa computer 36 or a network telephone 38 (which is a telephone that isfitted with a network interface card for communications over packet datanetworks). Examples of packet data communications include web browsing,electronic mail, text chat sessions, file transfers, interactive gamesessions, voice-over-IP (Internet Protocol) sessions, and so forth.

The wireless communications network 10 thus provides two different typesof communications: circuit-switched communications and packet-switchedcommunications. Circuit-switched communications are routed through theMSC 24, while packet-switched communications are routed through the PDSN30. In circuit-switched communications, a dedicated end-to-end channelis established for the duration of a call session. However,packet-switched communications utilize a connectionless intranetworklayer, such as that defined by the Internet Protocol (IP). Inpacket-switched communications, packets or other units of data carryrouting information (in the form of addresses) that is used to route thepackets or data units over one or more paths to a destination endpoint.

One version of IP, referred to as IPv4, is described in Request forComments (RFC) 791, entitled “Internet Protocol,” dated September 1981;and another version of IP, referred to as IPv6, is described in RFC2460, “Internet Protocol, Version 6 (IPv6) Specification,” datedDecember 1998.

The base station 14 is part of a first type of wireless communicationssystem, which in one example is a 1xRTT system. 1xRTT refers to onephase of the CDMA 2000 technology. More generally, the base station 14is referred to as an IS-2000 base station.

The wireless communications network 10 also includes a second type ofwireless communications system, which in the described example is a1xEV-DO or 1xEV system. One version of 1xEV-DO is defined in theTIA/EIA/IS-856 Standard, entitled “CDMA 2000 High Rate Packet Data AirInterface Specification.” Note that although only one base station 14and one access network 40 are shown in FIG. 1, the wirelesscommunications network 10 can include plural base stations 14 and pluralaccess networks 40.

The first type of wireless communications system (e.g., 1xRTT system)supports packet data services at a first performance level while thesecond type of wireless communications system (e.g., 1xEV-DO system)supports packet data services at a second, higher performance level. Asused here, the term “performance level” refers to data bandwidth, datarate, and/or any other parameter that indicates a level of performancein wireless communications. Thus, generally, the first and second typesof wireless communication systems within the wireless communicationsnetwork 10 provide different performance levels for packet dataservices.

The 1xEV-DO wireless communications system includes an access network(AN) 40, which is network equipment that provides data connectivitybetween a packet-switched data network (such as the data network 34through the PDSN 30) and a mobile station (e.g., mobile station 43). Theaccess network 40 provides coverage in a cell 41. According to 1xEV-DO,a mobile station is referred to as an access terminal (AT), which is adevice that provides data connectivity to a user. However, as used here,“mobile station” refers to any user device, whether it is capable ofcommunicating in a 1xRTT wireless system, 1xEV-DO wireless system, orother wireless system. Examples of mobile stations include personaldigital assistants (PDAs), mobile telephones, mobile units coupled toportable computing devices, and so forth.

The access network 40 is equivalent to the base station 14 in the 1xRTT(or more generally, IS-2000) wireless system. The access network 40includes an access network controller (ANC) 42 and an access point (AP)44. As used here, “base station” or “base station system” refers to thebase station 14, the access network 40, or any other radio networkcontroller and/or radio transceiver that supports wirelesscommunications with mobile stations.

A packet control function (PCF) module 46 is coupled between the accessnetwork 40 and the PDSN 30. An interface 45 between the access network40 and the PCF module 46 may include A8 and A9 interfaces, and aninterface 47 between the PCF module 46 and the PDSN 30 may include A10and A11 interfaces. During a communications session, packet data isrouted between the access terminal 43 and another endpoint through theaccess network 40, PCF module 46, and PDSN 30.

In accordance with some embodiments of the invention, a relativelygraceful handoff procedure is provided for handing off a call between afirst type of wireless system and a second type of wireless system inresponse to a mobile station crossing a boundary between a cell coveredby the first type of wireless system and a cell covered by the secondtype of wireless system. In the arrangement illustrated in FIG. 1, themobile stations 43 and 16 are assumed to be “hybrid” mobile stationsthat are able to support communications in both the first and secondtypes of wireless systems. In one example, the hybrid mobile stations 43and 16 are capable of supporting communications in both 1xRTT and1xEV-DO wireless systems.

To support handoff of a call when one of the mobile stations 16 and 43crosses between one cell (covered by the first type of wireless system)and another cell (covered by the second type of wireless system), aninterface is defined between the access network 40 and the base station14. In the example of FIG. 1, this interface is referred to as an Ayinterface 50, which enables network-initiated handoff procedures.

According to one embodiment, the handoff performed is a hard handoff inwhich the call is maintained (that is, the call is not dropped andre-originated). A point-to-point protocol (PPP) connection between themobile station and the PDSN 30 is maintained through the use of apermanent terminal identifier, referred to as an international mobilesubscriber identity (IMSI). PPP is described in RFC 1661, entitled“Point-to-Point Protocol,” dated July 1994.

To enable handoffs between an access network 40 and a base station 14 inthe two different types of wireless systems, each of the access network40 and base station 14 is able to detect that a mobile station isapproaching the edge of the coverage area. Further, the access network40 has information about neighboring 1xRTT (or more generally, IS-2000)pilots, and vice versa, the base station 14 has information aboutneighboring 1xEV-DO pilots.

Generally, when the access network 40 detects that a mobile station isapproaching a coverage boundary, the access network 40 redirects themobile station to the neighboring IS-2000 base station 14. The accessnetwork 40 also initiates a handoff procedure (network-initiated handoffprocedure) by sending a message to notify the IS-2000 base station 14that a handoff is required. This message includes the mobile station'spermanent terminal identifier (IMSI) and a mobility event indicator.When the IS-2000 base station 14 receives the handoff notification, thebase station 14 initiates procedures to transfer connections to theIS-2000 wireless system.

A similar procedure is performed in the reverse direction when a mobilestation transitions from the coverage area of an IS-2000 base station 14to the coverage area of a 1xEV-DO access network 40.

Referring to FIG. 2, a message flow of the process of a handoff betweenthe access network 40 and the base station 14 is illustrated. For thesake of simplicity, the access network 40 and associated PCF module 46are shown as a unit, while the base station 14 and PCF module 32 areshown as a single unit.

An active 1xEV-DO data session 100 has been established between themobile station and the PDSN 30. In one example, the data session can bea voice-over-IP session, which is a session in which voice is carried inIP packets between two endpoints (a mobile station and another endpoint,such as an endpoint coupled to the data network 34). Other types ofsessions for carrying other types of data can also be established, suchas data communicated during web browsing, electronic mail, text chatsessions, file transfers, interactive gaming sessions, and so forth.

As noted above, IP defines a packet-switched protocol in which packetsare routed using addresses carried in the packets between endpoints. AnIP network includes routers that route packets based on source anddestination IP addresses carried in the packets. For voice-over-IP callsessions, voice data is carried as payload in IP packets. In oneexample, the voice data is stored in Real-Time Protocol (RTP) format.RTP defines a protocol for transporting real-time data, and is describedin RFC 1889, entitled “RTP: A Transport Protocol for Real-TimeApplications,” dated January 1996.

As the mobile station (43 in the illustrated example of FIG. 1) movestoward the boundary of the cell 41, the access network 40 detects acondition that a hard handoff is required. This can be based on pilotstrength measurements, for example. In response to this condition, theaccess network 40 sends a Handoff Required message (at 102) to thetarget base station 14 through the interface 50. The access network 40inserts the PANID (previous access network identifier) of the source PCFmodule 46 in the Handoff Required message.

The Handoff Required message indicates that for a given mobile stationthat already has a dedicated radio resource assigned, a handoff isrequired for the reason given by a “Cause” information element.Conventionally, a Handoff Required message is sent by an IS-2000 basestation to an MSC. In one embodiment, the Handoff Required messageexchanged over the Ay interface 50 between the access network 40 andbase station 14 is similar to Handoff Required messages exchangedbetween an IS-2000 base station and MSC, except that informationelements are provided to indicate that the source system is a 1xEV-DOsystem.

The base station 14 then forwards the Handoff Required message (at 104)to the MSC 24 with an indication that the Handoff Required message is arelayed message requesting handoff from the 1xEV-DO access network 40.For example, this can be indicated in the “Cause” information element ofthe Handoff Required message. The message provides the MSC with a listof target candidate cells or optional measurement information for theMSC to use to determine a target with an available radio channel. Uponreceiving a Handoff Required message, the MSC 24 may construct acandidate target list, modify an existing one, or use the existing listas received.

The MSC 24 then sends (at 106) a Handoff Request message to the targetbase station 14 to indicate that the mobile station is to be handed overto the base station 14. Upon receipt of the Handoff Request message, thetarget base station 14 allocates suitable idle radio resources.

The target base station also returns (at 110) a Handoff RequestAcknowledge message to the MSC 24 with appropriate RF channelinformation to allow the mobile station to be instructed to tune to thenew RF channel.

The MSC 24 then sends a Handoff Command message (at 111) to the targetbase station 14. The Handoff Command commences source cell handoffprocedures. Note that, conventionally, the Handoff Command message issent from the MSC to the source base station directly; however, inaccordance with one embodiment, the MSC 24 sends the Handoff Commandmessage to the target base station 14 in its role as proxy for thesource access network 40.

In response, the target base station 14 then sends a Handoff Commandmessage (at 112) to the source access network 40 over the Ay interface50 containing the appropriate RF channel information. Upon receipt ofthe Handoff Command message, the source access network 40 instructs themobile station to re-direct to the IS-2000 base station by sending (at114) a Handoff Direction message.

The access network 40 next sends (at 118) a Handoff Commenced message tothe target base station 14 over the Ay interface 50. The HandoffCommenced message is sent by the source access network 40 to the IS-2000base station 14 to indicate that the Handoff Direction message has beensent to the mobile station and that the mobile station is not expectedto return to the source access network 40. Note that, conventionally,the Handoff Command message is sent by the source base station to theMSC, but in the call flow of FIG. 2, the Handoff Command message is sentfrom the source access network 40 to the target base station 14.

The mobile station then sends (at 120) an Origination message to thetarget base station 14. The target base station 14 responds (at 122)with a Traffic Channel Assignment message.

Next, the target PCF module (in this case PCF module 32) sends (at 126)an A11 Registration Request message to the PDSN 30. The A11 RegistrationRequest message contains the Mobility Event Indicator within theVendor/Organization Specific Extension information element to the PSDN30. The Mobility Event Indicator is used during dormant and active/hardhandoffs. The A11 Registration Request message also includes the PANID(of the source PCF module 46) and the current access network identifier(CANID) (of the target PCF module 32) within the Vendor/OrganizationSpecific Extension information element. The A11 Registration Requestmessage is sent by the target PCF module 32 to the PDSN 30 after a shortdata burst has been successfully sent to the PDSN 30 from the target PCF32, with the A11 Registration Request message sent to establish an A10connection between the PCF module 32 and PDSN 30. The A11 RegistrationRequest message is described in RFC 2002, entitled “IP MobilitySupport,” dated October 1996.

The PDSN 30 validates the A11 Registration Request message and acceptsthe connection by returning (at 128) an A11 Registration Reply messagewith an accept indication to the target PCF 32. The accept indication isprovided in the “Code” information element of the A11 Registration Replymessage. In response, the A10 connecting binding information at the PDSN30 is updated to point to the target PCF module 32.

The target base station 14 detects that the mobile station hassuccessfully accessed the target. The target base station 14 then sends(at 130) a Handoff Complete message to the MSC 24. This indicates to theMSC 24 that the mobile station has arrived on the new RF channel and hascompleted connection procedures. At this point, a 1xRTT session isestablished (at 132) between a mobile station and the PDSN 30 (alsoreferred to as the A10 connection).

The PDSN 30 initiates termination of the A10 connection with the sourcePCF module (in this example PCF module 46) by sending (at 138) an A11Registration Update message. The A11 Registration Update message is sentto update the status of the A10 connection, in this case to terminatethe A10 connection. The source PCF module 46 responds (at 140) with anA11 Registration Update Acknowledge message.

The source PCF module 46 then sends (at 142) an A11 Registration Requestmessage with the Lifetime information element set to zero to tear downthe A10 connection. Also, accounting related information may be carriedin the A11 Registration Request message sent at 142. The PDSN 30 storesthe accounting related information for further processing beforereturning (at 144) an A11 Registration Reply message. The source PCFmodule 46 closes the A10 connection for the mobile station, which hasbeen handed off to the target base station 14.

At this point, all maintenance procedures that are required of thetarget PCF module 32 continue.

The message flow for transitioning from a 1xRTT base station to a1xEV-DO access network is similar to the message flow for transitioningfrom a 1xEV-DO access network to a 1xRTT base station, with somemodifications. To initiate the handoff, the source 1xRTT base stationalso sends a Handoff Required message to the target 1xEV-DO accessnetwork. The 1xEV-DO access network then waits for the mobile station tosend a Unicast Access Terminal Identifier (UATI)-Request message. Theaccess network responds with a UATI-Assignment message in response, andthen places the mobile station on the traffic channel. This is similarto the 1xEV-DO to 1xRTT transition, in which the network waits for themobile station to show up. However, exchanges of messaging (104, 106,110, 111, 130 in FIG. 2) between the target 1xEV-DO access network andthe MSC do not occur.

The remaining flow of messages are identical to that shown in FIG. 2,except that the roles of the 1xRTT base station and 1xEV-DO accessnetwork are reversed. However, instead of a Handoff Complete messagefrom the target to the MSC (130 in FIG. 2), the source 1xRTT basestation sends an indication to the MSC that the connection has beenhanded off to a 1xEV-DO access network (and thus the MSC need no longerbe involved in communications with the transitioning mobile station).

The MSC also sends a Clear Command message to the source 1xRTT basestation to start a clearing procedure used in hard handoffs to releasethe source RF channel and terrestrial resource. In response to the ClearCommand, the 1xRTT base station sends a Clear Complete message to theMSC.

Referring to FIG. 3, some components inside the access network 40/basestation 14 and PCF module 46 or 32 according to one example areillustrated. It is noted that FIG. 3 provides a functional view of some(but not all) of the components of the access network/base station. Theaccess network/base station and PCF module may be implemented on oneplatform or on multiple platforms, referred to as wireless packetservice platform(s) 200.

The access network/base station has a communications stack forcommunication over an air interface. The communications stack is made upof several layers, including a physical layer 202, a medium accesscontrol (MAC) layer 204, a link access control (LAC) layer 206, and alayer 3 signaling block 208. The physical layer 202 implements thephysical layer protocol that provides channel structure, frequency,power output, and modulation specifications for the forward and reverselinks in the air interface.

The MAC layer 204 implements the medium access protocol and is responsefor transport of LAC protocol data units using services provided by thephysical layer 202. The LAC layer 206 implements a data link protocolthat provides for the correct transport and delivery of signalingmessages generated by the layer 3 signaling block 208. The layer 3signaling block 208 provides the control messaging for wirelesscommunications between the access network/base station and the mobilestation. The access network/base station may also include other layers210. In addition, one or more applications 212 may be executable in theaccess network/base station.

To communicate with the PCF module, the access network/base stationincludes A8 and A9 interfaces 214. As discussed above, the A8 interfacecarries user traffic between the base station or access network and thePCF module, and the A9 interface carries signaling between the basestation or access network and the PCF module. Also, the accessnetwork/base station includes an Ay interface 219 to enablecommunication between a base station and access network.

The various components of the base station or access network illustratedin FIG. 3 can be implemented as software components, hardwarecomponents, or a combination thereof. The parts that are implemented insoftware are executable on a control unit 216 (which may be implementedas a single processor or as multiple processors). The control unit 216is coupled to a storage unit 218 for storing data and instructions ofsoftware.

The PCF module includes A8 and A9 interfaces 220, as well as signalingand user traffic control module 222. Other layers may also be present inthe PCF module, which are not shown. The signaling and user trafficcontrol module 222 also communicates with A10/A11 interface 224. Asnoted above, the A10 interface carries user traffic between the PCFmodule and the PDSN 30, and the A11 interface carries signaling betweenthe PCF module and the PDSN 30.

The components of the PCF module may be implemented as software,hardware, or a combination thereof. Parts that are implemented insoftware are executable on a control unit 226, which is coupled to astorage unit 228 in the PCF module.

Each control unit referred to herein includes a microprocessor, amicrocontroller, a processor card (including one or more microprocessorsor microcontrollers), or other control or computing devices. The storageunits referred to in this discussion include one or moremachine-readable storage media for storing data and instructions. Thestorage media include different forms of memory including semiconductormemory devices such as dynamic or static random access memories (DRAMsor SRAMs), erasable and programmable read-only memories (EPROMs),electrically erasable and programmable read-only memories (EEPROMs) andflash memories; magnetic disks such as fixed, floppy and removabledisks; other magnetic media including tape; and optical media such ascompact disks (CDs) or digital video disks (DVDs). Instructions thatmake up the various software routines or modules in the various devicesor systems are stored in respective storage devices. The instructionswhen executed by a respective control unit cause the corresponding nodeor system to perform programmed acts.

The instructions of the software routines or modules are loaded ortransported to each node or system in one of many different ways. Forexample, code segments including instructions stored on floppy disks, CDor DVD media, a hard disk, or transported through a network interfacecard, modem, or other interface device are loaded into the device orsystem and executed as corresponding software routines or modules. Inthe loading or transport process, data signals that are embodied incarrier waves (transmitted over telephone lines, network lines, wirelesslinks, cables, and the like) communicate the code segments, includinginstructions, to the device or system. Such carrier waves are in theform of electrical, optical, acoustical, electromagnetic, or other typesof signals.

As used here, a “controller” refers to a hardware component, softwarecomponent, or a combination of the two. Although used in the singularsense, a “controller” can also refer to plural hardware components,plural software components, or a combination thereof.

While the invention has been disclosed with respect to a limited numberof embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover such modifications and variations as fall within the truespirit and scope of the invention.

1. A method of performing wireless communications, comprising:communicating bearer traffic for a packet-switched communicationssession from a first base station associated with a first type ofwireless system to a mobile station; determining, by the first basestation, if handoff is required from the first base station to a secondbase station associated with a second, different type of wirelesssystem; and in response to determining that the handoff is required,sending a message from the first base station to the second base stationover an interface between the first base station and second basestation, the message indicating to the second base station that handoffis required.
 2. The method of claim 1, wherein the first base stationcomprises an IS-2000 base station, and wherein communicating the bearertraffic comprises communicating the bearer traffic from the IS-2000 basestation to the mobile station.
 3. The method of claim 2, whereindetermining if handoff is required from the first base station to thesecond base station comprises determining if handoff is required fromthe IS-2000 base station to a 1xEV access network.
 4. The method ofclaim 2, wherein determining if handoff is required from the first basestation to the second base station comprises determining if handoff isrequired from the IS-2000 base station to a High Data Rate (HDR) accessnetwork.
 5. The method of claim 1, wherein the first base stationcomprises a High Data Rate access network, and wherein communicating thebearer traffic comprises communicating the bearer traffic from the HighData Rate access network to the mobile station.
 6. The method of claim5, wherein determining if handoff is required from the first basestation to the second base station comprises determining if handoff isrequired from the High Data Rate access network to an IS-2000 basestation.
 7. The method of claim 1, wherein the first base stationcomprises a 1xEV access network, and wherein communicating the bearertraffic comprises communicating the bearer traffic from the 1xEV accessnetwork to the mobile station.
 8. The method of claim 7, whereindetermining if handoff is required from the first base station to thesecond base station comprises determining if handoff is required fromthe 1xEV access network to a 1xRTT base station.
 9. The method of claim1, further comprising receiving another message from the second basestation at the first base station to initiate a handoff procedure. 10.The method of claim 9, further comprising sending a further message fromthe first base station to the second base station to indicate that themobile station has been directed to hand off to the second base station.11. The method of claim 1, wherein sending the message comprises sendingthe message over a link between the first base station and the secondbase station.
 12. The method of claim 1, further comprising performing ahard handoff from the first base station to the second base station. 13.A first base station system that performs wireless communications with amobile station according to a first protocol, the first base stationsystem comprising: an interface to a second base station system thatperforms wireless communications with the mobile station according to asecond, different protocol; and a controller to communicate bearertraffic for a packet-switched communications session with the mobilestation, the controller to further determine that the handoff isrequired from the first base station to the second base station, and inresponse to determining the handoff is required, send messaging to thesecond base station system through the interface to perform a handoff ofthe packet-switched communications session from the first base stationsystem to the second base station system.
 14. The first base stationsystem of claim 13, wherein the controller is to perform the handoff byperforming a hard handoff.
 15. The first base station system of claim13, wherein the controller is to communicate bearer traffic according toa 1xRTT format with the mobile station.
 16. The first base stationsystem of claim 15, wherein the second base station system comprises a1xEV base station, and wherein the controller is to send the messagingto the 1xEV base station.
 17. The first base station system of claim 13,wherein the controller is to send the messaging by sending a messageindicating that a handoff is required to the second base station systemthrough the interface.
 18. The first base station system of claim 17,wherein the controller is to further receive a message initiating thehandoff procedure.
 19. The first base station system of claim 18,wherein the controller is to send a further message from the first basestation system to the second base station system to indicate that themobile station has been directed to hand off to the second base stationsystem.
 20. An article comprising at least one storage medium containinginstructions that when executed cause a first base station system to:communicate signaling according to a first protocol with a mobilestation to establish a packet-switched communications session betweenthe mobile station and another endpoint; determine if a handoff isrequired to a second base station system that performs wirelesscommunications with the mobile station according to a second, differentprotocol; and in response to determining that the handoff is required,send messaging to the second base station system through a link betweenthe first and second base station systems to perform the handoff. 21.The article of claim 20, wherein the first base station comprises a1xRTT base station, and wherein the instructions when executed cause thefirst base station system to communicate 1xRTT signaling with the mobilestation.
 22. The article of claim 21, wherein the instructions whenexecuted cause the first base station system to determine if handoff isrequired by determining if handoff is required from the 1xRTT basestation to one of a 1xEV access network and a High Data Rate (HDR)access network.
 23. The article of claim 20, wherein the first basestation comprises one of a High Data Rate (HDR) access network and a1xEV access network, and wherein the instructions when executed causethe first base station system to communicate one of High Data Rate (HDR)signaling and 1xEV signaling with the mobile station.
 24. The article ofclaim 23, wherein the instructions when executed cause the first basestation system to determine if handoff is required by determining ifhandoff is required from the one of the High Data Rate (HDR) accessnetwork and 1xEV access network to a 1xRTT base station.
 25. The articleof claim 20, wherein the instructions when executed cause the first basestation system to send messaging by sending a message to the second basestation system indicating that a handoff is required.
 26. The method ofclaim 1, wherein sending the message comprises sending the message overa link that directly connects the first base station and second basestation.
 27. The apparatus of claim 13, wherein the interface allows themessaging to be sent from the first base station system directly to thesecond base station system.
 28. The article of claim 20, wherein sendingthe messaging to the second base station through the link comprisessending the messaging to the second base station through the link thatdirectly connects the first base station system to the second basestation system.
 29. The method of claim 1, wherein the mobile stationcomprises a hybrid mobile station that is able to support at least twodifferent wireless communications protocols including a first wirelesscommunications protocol and a second wireless communications protocol,wherein determining if the handoff is required from the first basestation to the second base station comprises determining if the handoffis required from the first base station that communicates with thehybrid mobile station according to the first wireless communicationsprotocol, to the second base station that communicates with the hybridmobile station according to the second wireless communications protocol.30. The method of claim 29, wherein the first wireless communicationsprotocol comprises a 1xEV protocol, and the second wirelesscommunications protocol comprises a 1xRTT protocol.
 31. The apparatus ofclaim 13, wherein the mobile station comprises a hybrid mobile stationthat is able to perform wireless communications according to both thefirst and second protocols, the controller to communicate the bearertraffic with the hybrid mobile station.
 32. The apparatus of claim 31,wherein the first protocol comprises a 1xEV protocol, and the secondprotocol comprises a 1xRTT protocol.
 33. The article of claim 20,wherein exchanging the signaling with the mobile station comprisesexchanging the signaling with a hybrid mobile station that is able toperform wireless communications according to both the first and secondprotocols.
 34. The article of claim 33, wherein the first protocolcomprises a 1xEV protocol, and the second protocol comprises a 1xRTTprotocol.